Method for the electrochemical deposition of carbon nanotubes

ABSTRACT

This invention relates to the electrochemical deposition of carbon nanotubes (“CNTs”) on a substrate using an electrochemical cell. A dispersion of a complex of CNTs and an anionic polymer is neutralized and thereby caused to deposit on the anode plate of the cell.

This application claims priority under 35 U.S.C. §119(e) from, andclaims the benefit of, U.S. Provisional Application No. 61/032,505,filed Feb. 29, 2008, which is by this reference incorporated in itsentirety as a part hereof for all purposes.

TECHNICAL FIELD

This invention relates to the electrochemical deposition of carbonnanotubes (“CNTs”) on a substrate.

BACKGROUND

Carbon nanotubes are well known to have unique and useful electricalproperties, and are frequently used in the fabrication of the cathode ofa field emission device (“FED”). However, adoption of these materials isconstrained by their high cost

US 2006/0063464 describes the deposition of carbon nanotubes byelectrochemical methods. A need remains, however, for methods for theelectrodeposition of carbon nanotubes that produce electron fieldemitters with good uniformity and low material consumption, and in whicha relatively high emission current is consistently obtained from arelatively low voltage input.

SUMMARY

One objective of this invention is thus to provide a method for making auniform CNT film on a substrate such as a conducting substrate with gooduniformity and low material consumption. Another objective is to providea method for making a CNT film that, when used as electron fieldemitter, consistently produces a relatively high emission current from arelatively low voltage input A further objective is to provide from thismethod a CNT film that may be easily patterned for use in electronicapplications. The CNT film so patterned may be used, for example, in acathode assembly that is installed in a field emission device.

One embodiment of this invention thus provides a method for thedeposition of carbon nanotubes by (a) providing an electrochemical cellthat comprises a cathode, an anode plate, a first electricallyconducting pathway connecting the cathode to an electrical power supply,and a second electrically conducting pathway connecting the electricalpower supply to the anode plate; (b) providing as an aqueous electrolytedisposed between the cathode and the anode a dispersion of a complexformed from carbon nanotubes and a first anionic polymer; (c) applying avoltage to the electrochemical cell to deposit the complex on the anode;and (d) removing the anode plate from the electrochemical cell andfiring the plate in air.

In another embodiment, this invention provides a method for thedeposition of an electron emitting material on a substrate by (a)providing an electrochemical cell that comprises a cathode, an anodeplate, a first electrically conducting pathway connecting the cathode toan electrical power supply, and a second electrically conducting pathwayconnecting the electrical power supply to the anode plate; (b) providingan aqueous electrolyte disposed between the cathode and the anode,wherein the electrolyte comprises boric acid and/or a borate compound,and a dispersion of a complex formed from carbon nanotubes and a firstanionic polymer; and (c) applying a voltage to the electrochemical cellto deposit the complex on the anode.

In a further embodiment, this invention provides a film that includes asubstrate and, disposed on the substrate, (a) boric acid and/or a boratecompound, and (b) a complex formed from carbon nanotubes and a firstanionic polymer. Alternatively, in this embodiment, there may further bedisposed on the substrate coagulant residue.

In yet another embodiment, this invention provides a method for thedeposition of an electron emitting material on a substrate, by (a)depositing an electron emitting material on a substrate to prepare anelectron field emitter; (b) installing the electron field emitter as theanode plate in an electrochemical cell, wherein the electrochemical cellfurther comprise a cathode, a first electrically conducting pathwayconnecting the cathode to an electrical power supply, and a secondelectrically conducting pathway connecting the electrical power supplyto the anode plate; (c) providing an electrolyte disposed between thecathode and the anode plate that comprises boric acid and/or a boratecompound; and (d) applying a voltage to the electrochemical cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic representation of the mechanism of deposit inone embodiment of the methods of this invention.

FIG. 2 shows the configuration of an electrochemical cell as used in theexamples.

FIG. 3 shows a plot of the results of Example 1.

FIG. 4 shows a plot of the results of Example 2.

DETAILED DESCRIPTION

A CNT film is made by the method of this invention by the deposition ofCNTs on a substrate by electrochemical means, and for such purpose themethod hereof involves the use of an electrochemical cell. The cellcontains a cathode, an anode plate, a first electrically conductingpathway connecting the cathode to an electrical power supply, and asecond electrically conducting pathway connecting the electrical powersupply to the anode plate. An aqueous electrolyte is provided to thecell and is disposed between the anode plate and the cathode. Containedin the electrolyte is a dispersion of a complex formed from CNTs and afirst anionic polymer, and optionally a coagulant.

CNTs as used herein are generally about 0.5-2 nm in diameter where theratio of the length dimension to the narrow dimension, i.e. the aspectratio, is at least 5. In general, the aspect ratio is between 10 and2000. CNTs are comprised primarily of carbon atoms, however may be dopedwith other elements, e.g. metals. The carbon-based nanotubes of theinvention can be either multi-walled nanotubes (MWNTs) or single-wallednanotubes (SWNTs). A MWNT, for example, includes several concentricnanotubes each having a different diameter. Thus, the smallest diametertube is encapsulated by a larger diameter tube, which in turn, isencapsulated by another larger diameter nanotube. A SWNT, on the otherhand, includes only one nanotube.

CNTs may be produced by a variety of methods, and are additionallycommercially available. Methods of CNT synthesis include laservaporization of graphite [A. Thess et al, Science 273, 483 (1996)], arcdischarge [C. Journet et al, Nature 388, 756 (1997)] and HiPCo (highpressure carbon monoxide) process [P. Nikolaev et al, Chem. Phys. Lett.313, 91-97 (1999)]. Chemical vapor deposition (CVD) can also be used inproducing carbon nanotubes [J. Kong et al, Chem. Phys. Lett. 292,567-574 (1998); J. Kong et al, Nature 395, 878-879 (1998); A. Cassell etal, J. Phys. Chem. 103, 6484-6492 (1999); H. Dai et al, J. Phys. Chem.103, 11246-11255 (1999)]. Additionally CNTs may be grown via catalyticprocesses both in solution and on solid substrates [Yan Li et al, Chem.Mater.; 2001; 13(3); 1008-1014); (N. Franklin and H. Dai, Adv. Mater.12, 890 (2000); A. Cassell et al, J. Am. Chem. Soc. 121, 7975-7976(1999)].

A major obstacle to the use of CNTs has been the diversity of tubediameters, chiral angles, and aggregation states in nanotube samplesobtained from the various preparation methods. Aggregation isparticularly problematic because the highly polarizable, smooth-sidedfullerene tubes readily form parallel bundles or ropes with a large vander Waals binding energy. This bundling perturbs the electronicstructure of the tubes, and it confounds almost all attempts to separatethe tubes by size or type or to use them as individual macromolecularspecies.

There is thus provided by this invention a method for dispersing apopulation of bundled carbon nanotubes by contacting the bundlednanotubes with an aqueous solution of an anionic polymer. A complexcontaining the anionic polymer and the CNTs is thereby formed, but theassociation between the anionic polymer and the CNTs in the complex is aloose association, is formed essentially by van der Waals bonds or someother non-covalent means, and is not formed through the interaction ofspecific functionalized groups. The structural integrity of the CNTs istherefore retained, but the complexes they form with the anionicpolymers become, when present in the electrolyte, suspended in adispersion in the electrolyte.

A variety of anionic polymers may thus be used as dispersants for thepurpose of dispersing CNTs in an aqueous solution by facilitating theformation of the polymer/CNT complex, but a preferred polymer for usefor such purpose is a nucleic acid, particularly a stabilized solutionof nucleic acid molecules. Nucleic acids are very effective indispersing CNTs by the formation of nanotube-nucleic acid complexesbased on non-covalent interactions between the nanotube and the nucleicacid molecule. The method of this invention therefore includes a methodfor the dispersion of bundled CNTs by contacting the nanotubes with asolution of anionic polymers such as nucleic acid molecules.

In the following discussion of the use of nucleic acid molecules to formcomplexes with and thereby disperse CNTs, the following terms andabbreviations are used:

“cDNA” means complementary DNA

“PNA” means peptide nucleic acid

“SEM” means scanning electron microscopy

“ssDNA” means single stranded DNA

“tRNA” means transfer RNA

“CNT” means carbon nanotube

“MWNT” means multi-walled nanotube

“SWNT” means single walled nanotube

“TEM” means transmission electron microscopy

A “nucleic acid molecule” is defined as a polymer of RNA, DNA, orpeptide nucleic acid (PNA) that is single- or double-stranded,optionally containing synthetic, non-natural or altered nucleotidebases. A nucleic acid molecule in the form of a polymer of DNA may becomprised of one or more segments of cDNA, genomic DNA or synthetic DNA.

The letters “A”, “G”, “T”, “C” when referred to in the context ofnucleic acids will mean the purine bases adenine (C₅H₅N₅) and guanine(C₅H₅N₅O) and the pyrimidine bases thymine (C₅H₆N₂O₂) and cytosine(C₄H₅N₃O), respectively.

The term “peptide nucleic acids” refers to a material having stretchesof nucleic acid polymers linked together by peptide linkers.

A “stabilized solution of nucleic acid molecules” refers to a solutionof nucleic acid molecules that are solubilized and in a relaxedsecondary conformation.

A “nanotube-nucleic acid complex” means a composition comprising acarbon nanotube loosely associated with at least one nucleic acidmolecule. Typically the association between the nucleic acid and thenanotube is by van der Waals bonds or some other non-covalent means.

The term “agitation means” refers to a devices that facilitate thedispersion of nanotubes and nucleic acids. A typical agitation means issonication.

The term “denaturant” refers to substances effective in the denaturationof DNA and other nucleic acid molecules.

Standard recombinant DNA and molecular biology techniques used here arewell known in the art and are described, for example, by Sambrook,Fritsch and Maniatis, Molecular Cloning: A Laboratory Manual, SecondEdition, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.(1989) (hereinafter “Maniatis”); by Silhavy, Bennan and Enquist,Experiments with Gene Fusions, Cold Spring Harbor Laboratory Cold PressSpring Harbor, N.Y. (1984); and by Ausubel et al, Current Protocols inMolecular Biology, published by Greene Publishing Assoc. andWiley-Interscience (1987).

Nucleic acid molecules as used in a method of this invention may be ofany type and from any suitable source, and include without limitationDNA, RNA and peptide nucleic acids. The nucleic acid molecules usedherein may be generated by synthetic means or may be isolated fromnature by protocols known in the art (see, e.g., Sambrook supra). Thenucleic acid molecules may be either single stranded or double strandedand may optionally be functionalized at any point with a variety ofreactive groups, ligands or agents. Functionalization of nucleic acidsis not, however, required for their association with CNTs for thepurpose of dispersion, and most of the nucleic acids used herein fordispersion do lack functional groups and are therefore referred toherein as “unfunctionalized”.

Peptide nucleic acids (PNA) are particularly useful herein fordispersion as they possess the double functionality of both nucleicacids and peptides. Methods for the synthesis and use of PNA's are knownin the art as discussed, for example, in Antsypovitch, Peptide nucleicacids: Structure, Russian Chemical Reviews (2002), 71(1), 71-83.

Nucleic acid molecules as used herein may have any composition of basesand may even consist of stretches of the same base (poly A or poly T forexample) without impairing the ability of the nucleic acid molecule todisperse the bundled CNTs. Preferably the nucleic acid molecules will beless than about 2000 bases where less than 1000 bases is preferred andwhere from about 5 bases to about 1000 bases is most preferred.Generally the ability of nucleic acids to disperse CNTs appears to beindependent of sequence or base composition, however there is someevidence to suggest that the less G-C and T-A base-pairing interactionsin a sequence, the higher the dispersion efficiency, and that RNA andvarieties thereof is particularly effective in dispersion and is thuspreferred herein. Nucleic acid molecules suitable for use herein includewithout limitation those having the general formula:

-   -   1. An wherein n=1-2000;    -   2. Tn wherein n=1-2000;    -   3. Cn wherein n=1-2000;    -   4. Gn wherein n=1-2000;    -   5. Rn wherein n=1-2000, and wherein R may be either A or G;    -   6. Yn wherein n=1-2000, and wherein Y may be either C or T;    -   7. Mn wherein n=1-2000, and wherein M may be either A or C;    -   8. Kn wherein n=1-2000, and wherein K may be either G or T;    -   9. Sn wherein n=1-2000, and wherein S may be either C or G;    -   10. Wn wherein n=1-2000, and wherein W may be either A or T;    -   11. Hn wherein n=1-2000, and wherein H may be either A or C or        T;    -   12. Bn wherein n=1-2000, and wherein B may be either C or G or        T;    -   13. Vn wherein n=1-2000, and wherein V may be either A or C or        G;    -   14. Dn wherein n=1-2000, and wherein D may be either A or G or        T; and    -   15. Nn wherein n=1-2000, and wherein N may be either A or C or T        or G.

In addition to the combinations listed above, any of these sequences mayhave one or more deoxyribonucleotides replaced by ribonucleotides (i.e.RNA or RNA/DNA hybrid) or one or more sugar-phosphate linkages replacedby peptide bonds (i.e. PNA or PNA/RNA/DNA hybrid).

Nucleic acid molecules as used herein may be stabilized in a suitablesolution. It is preferred that the nucleic acid molecules be in arelaxed secondary conformation and only loosely associated with eachother to allow for the greatest contact by individual strands with theCNTs. Stabilized solutions of nucleic acids are common and well known inthe art (see Sambrook, supra) and typically include salts and bufferssuch as sodium and potassium salts, and TRIS (Tris(2-aminoethyl)amine),HEPES (N-(2-hydroxyethyl)piperazine-N′-(2-ethanesulfonic acid), and MES(2-(N-Morpholino)ethanesulfonic acid. Preferred solvents for stabilizednucleic acid solutions are those that are water miscible where water ismost preferred. The process of dispersion may be improved with theoptional addition of nucleic acid denaturing substances to the solution.Common denaturants include but are not limited to formamide, urea andguanidine. A non-limiting list of suitable denaturants may be found inSambrook, supra.

To prepare a dispersion according to one embodiment of the methodhereof, an anionic polymer such as one or more nucleic acid moleculesmay be contacted with a population of bundled carbon nanotubes. It ispreferred, although not required, that contact be made in the presenceof an agitation means of some sort. Typically the agitation meansemploys sonication, but may also include devices that produce high shearmixing of the nucleic acids and CNTs (i.e. homogenization), or anycombination thereof. Upon agitation, the CNTs will become dispersed andwill form nanotube-nucleic acid complexes comprising at least onenucleic acid molecule loosely associated with the CNT by hydrogenbonding or some other non-covalent means.

Temperature during the process of contacting CNTs with a nucleic acidmay have an effect on the efficacy of the dispersion. Mixing at roomtemperature or higher has been seen to give longer dispersion timeswhereas mixing at temperatures below room temperature (23° C.) has beenseen to give more rapid dispersion times, and temperatures of about 4°C. are preferred. The dispersion of CNTs by contact with nucleic acidmolecules is also described in US 2004/0132072 and US 2004/0146904, eachof which is by this reference incorporated in its entirety as a parthereof for all purposes.

In addition to the nucleic acid molecules described above, one or moreother anionic polymers may be used for the purpose of preparing anaqueous dispersion of CNTs. Examples of other anionic polymers that aresuitable for use in the preparation of a dispersion of CNTs includewithout limitation ionized poly(acrylic acid) (“PAA”) or ionizedethylene/(meth)acrylic acid copolymer (“EAA” or “EMAA”), either of whichmay be neutralized with cations such as Na⁺, K⁺, NH₄ ⁺ or Cr⁺; styrenicionomers such as styrene/sodium styrene sulfonate copolymer (PSS) orstyrene/sodium styrene methacrylate copolymer; and ionizedtetrafluoroethylene/sulfonic acid copolymers such as Nafion™ copolymer(from DuPont) in which the sulfonic acid group in atetrafluoroethylene/perfluorovinyl ether copolymer may be sodiumneutralized. As indicated above with respect to nucleic acid molecules,ultrasonication or other mixing means may be applied to facilitate thedispersion of CNTs in an aqueous solution of one or more of the anionicpolymers discussed above.

In one embodiment, deposition on the anode plate of the cell of thecomplexes formed from CNTs and molecules of an anionic polymer, asdispersed in the electrolyte solution contained in the cell, will befacilitated by the presence therein of the optional coagulant. Thecoagulant will neutralize the negative charge on the anionic polymer inthe complex. As the population of anionic polymer/CNT complexes has beenmaintained in dispersion primarily by the repulsion of one negativelycharged complex from another (or by the repulsion of positively chargeddouble layers surrounding the complexes), neutralization of thosenegative charges (or compression of the double layer) by the coagulantwill remove the force enabling the population of complexes to remain indispersion in the electrolyte solution. As the action of the coagulantto neutralize the complexes occurs in close proximity to the anodeplate, the complexes (as no longer dispersed) will in varying degreesundergo a transition from solution phase to solid phase, becomeprogressively aggregated and agglomerated (similar to the formation offloccules and flocs), and then be collected and deposited on the surfaceof the anode plate. In addition to the CNT complexes, the material asdeposited on the plate may include coagulant residue.

When first and second anionic polymers are present in the electrolytesolution, they may be, for example, a first polymer that forms a complexwith CNTs, and a second polymer that does not form a complex, or that ismore loosely associated with CNTs than the first polymer. The first andsecond polymers may become deposited on the surface of the anode at thesame time, and the first polymer may, for example, be deposited in amatrix of the second polymer. If additional materials, such asconductive or functionalized particles, are needed to enhance theusefulness and performance of the cell anode plate as a component in afield emission device are present in the electrolyte solution, thosematerials may be deposited on the anode plate at the same time as theanionic polymer/CNT complexes. FIG. 2 shows a typical example of thetype of film formed by such deposition on the anode plate, which filmhas good uniformity of evenly deposited, well-adhered material allacross its surface.

Coagulants suitable for use herein for the purpose of neutralizing ananionic polymer/CNT complex include inorganic coagulants such astrivalent cations formed from metals including Group VIII/VIIIA metalssuch as iron, cobalt, ruthenium or osmium. As a trivalent cation can beup to as much as ten times more effective in neutralizing the complexthan a divalent cation, a convenient way to provide the coagulant is tosupply a divalent cation such as tris(2,2′-bipyridyl)dichloro-ruthenium(II) to the electrolyte solution wherein the 2⁺ cation is oxidized to a3⁺ valence by the loss of electrons to the anode plate. A schematicrepresentative of this mechanism is shown in FIG. 1. In the case, forexample, of the use of a metal cation as the coagulant, coagulantresidue will thus be the cation as oxidized by interaction with ananionic polymer/CNT complex.

In an alternative embodiment, however, a coagulant is not used where theanode plate is formed from a metal such as silver or nickel. In suchcase, the metal on the plate dissolves in the electrolyte solution, andthe charge on an anionic polymer/CNT complex is neutralized by cationsformed from metal atoms that have gone into solution from the solidmetal from which the plate if formed.

In a further alternative embodiment, the electrolyte solution maycontain, in addition to one or more anionic polymers and the optionalcoagulant, boric acid and/or a borate compound. Borate compoundssuitable for use in the electrolyte solution include, for example, thoserepresented by the structural formula B—(—R³)(—R⁴)(—R⁵), wherein R³, R⁴and R⁵ may be the same or different, and each independently representsan alkyloxy group, an alkenyloxy group, an aryloxy group, an aralkyloxygroup or a halogen atom; and when R⁴ and R⁵ are an alkyloxy group, analkenyloxy group, an aryloxy group or an aralkyloxy group, R⁴ and R⁵ maybe combined to each other to form a cyclic structure together with theboron atom.

The alkyloxy group represented by R³, R⁴ or R⁵ may have a substituentand specifically, the alkyloxy group is preferably a substituted orunsubstituted, linear or branched alkyloxy group having from 1 to 10carbon atoms. Examples thereof include methoxy, ethoxy, propoxy,isopropoxy, butoxy, isobutoxy, sec-butoxy, pentyloxy, hexyloxy,heptyloxy, octyloxy, 3-methoxypropoxy, 4-chlorobutoxy and2-diethylaminoethoxy.

The alkenyloxy group represented by R³, R⁴ or R⁵ may have a substituentand specifically, the alkenyloxy group is preferably a substituted orunsubstituted, linear or branched alkenyloxy group having from 3 to 12carbon atoms. Examples thereof include a propenyloxy group, a butenyloxygroup, a pentenyloxy group, a hexenyloxy group, a heptenyloxy group, anoctenyloxy group, a dodecenyloxy group and prenyloxy group.

The aryloxy group represented by R3, R⁴ or R⁵ may have a substituent andspecifically, the aryloxy group is a substituted or unsubstitutedaryloxy group. Examples thereof include phenoxy, tolyloxy, xylyloxy,4-ethylphenoxy, 4-butylphenoxy, 4-tert-butylphenoxy, 4-methoxyphenoxy,4-diethylaminophenoxy, 2-methylphenoxy, 2-methoxyphenoxy, 1-naphthoxy,2-naphthoxy and 4-methylnaphthoxy.

The aralkyloxy group represented by R³, R⁴ or R⁵ may have a substituentand specifically, the aralkyloxy group is a substituted or unsubstitutedaralkyloxy group. Examples thereof include a benzyloxy group, aphenethyloxy group, a phenylpropyloxy group, a 1-naphthylmethyloxygroup, a 2-naphthylmethyloxy group and a 4-methoxybenzyloxy group.

Specific examples of a borate compound suitable for use herein include atrimethyl borate, a triethyl borate, a tri-n-propyl borate, atruisopropyl borate, a tri-n-butyl borate, a truisobutyl borate, atri-n-octyl borate, a butyldiethyl borate, anethyldi(2-phenethyl)borate, a triphenyl borate, adiethyl-4-methoxyphenyl borate, a diethylcyclohexyl borate,trichloroborane, trifluoroborane, diethoxychloroborane,n-butoxydichloroborane, and tris borate(ethylenediaminetetraaceticacid).

Specific examples of a compound having a cyclic structure containing aboron atom and two oxygen atoms within the ring formed by combining R⁴and R⁵ to each other include 2-methoxy-1,3,2-dioxaborinane,2-ethoxy-1,3,2-dioxaborolane, 2-butoxy-1,3,2-dioxaborinane,2phenoxy-1,3,2-dioxaborinane,2-phenoxy-4,4,6-trimethyl1,3,2-dioxaborinane,2-naphthoxy-1,3,2-dioxaborinane, 2-methoxy-1,3,2-benzodioxaborole and2-ethoxy-1,3,2-benzodioxaborin.

In this embodiment, boric acid and/or a borate compound may be used inthe electrolyte solution at a concentration therein in the range ofabout 0.1 wt % or more, or about 0.5 wt % or more, and yet about 10 wt %or less, or about 5 wt % or less.

In this embodiment of the method hereof, where boric acid and/or aborate compound is present in the electrolyte solution, the material asdeposited on the cell anode plate may thus include, in addition to theCNT complexes, residue of the optional coagulant and/or some of theboric acid and/or borate compound. In such event, a further embodimentof this invention includes a film that is composed of a substrate and,disposed or deposited on the substrate, a complex formed from carbonnanotubes and one or more anionic polymer(s), coagulant residue and/orboric acid and/or a borate compound.

As the plate that is used an the anode in the electrolytic cell willultimately be used in the cathode assembly of a field emission device,it is desirable that the plate as used in the cell already be providedwith conductive means onto which the CNTs may be deposited. One exampleof a suitable plate to use for such purpose is a glass plate, such as asoda lime glass plate, that is coated with a conductive material such asindium tin oxide (“ITO”). Alternatively, however, the plate used forsuch purpose could be a substrate on which conductive materials havefirst been deposited by thick film paste methods such as describedbelow.

The method hereof may be used to produce a film in which the depositedmaterial is deposited in a pre-determined pattern. This may beaccomplished by patterning the surface of the plate used as the cellanode using conventional photoimaging techniques. Thus a photoresist maybe activated through a mask and then developed to provide on the surfaceof the cell anode a pattern such as an array of circular wells. As theanionic polymer/CNT complexes are aggregated and settle out of solution,they are deposited only in the circular wells, and the photoresist maybe removed. This provides a patterned CNT film, with the anode plateserving as a substrate for the film, for use by installation in a fieldemission device.

The method hereof is generally performed by operation of theelectrochemical cell at lower potential such as less than about 5 volts,or from about 2 to less than about 5 volts, or from about 2 volts toabout 3 volts. Thickness of the deposited film is to a large extentdirectly related to length of deposition time. A deposition time in therange of about 1 to about 10 minutes, or in the range of about 1 toabout 2 minutes, may be used. A positive potential is maintained at thecell anode plate relative to the cathode of the cell.

After completion of the deposition of CNT complex material on the anodeplate in the cell, the plate may be removed from the cell, rinsed, driedand installed in such condition in a field emission device for use aspart of the cathode assembly therein to provide electron emission.Alternatively, however, before installation in a field emission device,the plate may be baked and/or fired to melt the deposited polymer(s) andutilize them in that form as an adhesive to more securely anchor theCNTs to the surface of the plate, resulting in a CNT-containing filmwith excellent abrasion resistance. Firing may be performed at atemperature in the range of about 250° C. to about 650° C., or about350° C. to about 550° C., or about 450° C. to about 525° C., for aperiod of time in the range of about 5 to about 30 minutes, or about 10to about 25 minutes, or about 10 to about 20 minutes, in an inert gassuch as nitrogen or in air.

After completion of the deposition of CNT complex material on the anodeplate in the cell, the plate may be installed in a field emission devicefor use as part of the cathode assembly therein to provide electronemission. When a voltage is applied to the CNTs, the anode of the deviceis bombarded with electrons. The anode of the field emission device isan electrode coated with an electrically conductive layer. When thefield emission device is used in a display device where the cathodecontains an array of pixels of the thick film paste deposits describedabove, the FED anode may comprise phosphors to convert incidentelectrons into light. The substrate of the FED anode would also beselected to be transparent so that the resulting light could betransmitted. From the cathode assembly and FED anode, a sealed unit isconstructed in which the cathode assembly and anode are separated byspacers, and there is an evacuated space between the anode and thecathode. This evacuated space is under partial vacuum so that theelectrons emitted from the cathode may transit to the anode with only asmall number of collisions with gas molecules. Frequently the evacuatedspace is evacuated to a pressure of less than 10⁻⁵ Torr.

Such a field emission device is useful in a variety of electronicapplications, e.g. vacuum electronic devices, flat panel computer andtelevision displays, back-light source for LCD displays, emission gateamplifiers, and klystrons and in lighting devices. For example, flatpanel displays having a cathode using a field emission electron source,i.e. a field emission material or field emitter, and a phosphor capableof emitting light upon bombardment by electrons emitted by the fieldemitter have been proposed. Such displays have the potential forproviding the visual display advantages of the conventional cathode raytube and the depth, weight and power consumption advantages of the otherflat panel displays. The flat panel displays can be planar or curved.U.S. Pat. Nos. 4,857,799 and 5,015,912 disclose matrix-addressed flatpanel displays using micro-tip cathodes constructed of tungsten,molybdenum or silicon. WO 94-15352, WO 94-15350 and WO 94-28571 discloseflat panel displays wherein the cathodes have relatively flat emissionsurfaces. These devices are also described in US 2002/0074932, which isby this reference incorporated in its entirety as a part hereof for allpurposes.

In an alternative embodiment of this invention, a field emission devicemay be made by preparing an electron field emitter by conventionalmeans. Such an electron field emitter would take the form of a substrateon which electron emitting material has been deposited, and would besuitable for use as, or to further prepare, a cathode assembly for usein an FED. The conventional means of preparing the electron fieldemitter would include, for example, depositing an electron emittingmaterial on a substrate by screen printing a thick film paste. After theelectron field emitter has been prepared, it is then installed as theanode plate in an electrolytic cell, as described elsewhere herein. Anaqueous electrolyte is provided to the cell and is disposed thereinbetween the cell cathode and the cell anode plate, which is thepreviously-prepared electron field emitter. Contained in the electrolyteis boric acid and/or a borate compound as described above. A voltage isthen applied to the cell, and the cell anode plate (thepreviously-prepared electron field emitter) is then removed from thecell.

In the preparation of an electron field emitter to be used in thisembodiment as the cell anode plate, there may be, for example, a depositon a substrate of a thick film paste containing an electron emittingmaterial. The electron emitting material contained in the thick filmpaste may be any acicular emitting material such as the CNTs describedabove, other forms of carbon such as carbon fibers, a semiconductor, ametal or mixtures thereof. Carbon fibers useful as an acicular emittingmaterial may be grown from the catalytic decomposition ofcarbon-containing gases over small metal particles are also useful asacicular carbon, and other examples of acicular carbon arepolyacrylonitrile-based (PAN-based) carbon fibers and pitch-based carbonfibers. As used herein, “acicular” means particles with aspect ratios of10 or more. Typically, glass frit, metallic powder or metallic paint ora mixture thereof is used to attach the electron emitting material tothe substrate in the electron field emitter to be used as, or in thepreparation of, a cathode assembly.

In a conventional attachment of an electron emitting material to asubstrate, various screen printing-type processes can be used. The meansof attachment must withstand and maintain its integrity under theconditions of manufacturing the apparatus into which the field emittingcathode is placed and under the conditions surrounding its use, e.g.typically vacuum conditions and temperatures up to about 450° C. Apreferred method is to screen print a paste comprised of the electronemitting material and glass frit, metallic powder or metallic paint or amixture thereof onto a substrate in the desired pattern and to then firethe dried patterned paste. For a wider variety of applications, e.g.those requiring finer resolution, the preferred process comprises screenprinting a paste which further comprises a photoinitiator and aphotohardenable monomer, photopatterning the dried paste and firing thepatterned paste.

The substrate can be any material to which the paste composition willadhere. If the paste is non-conducting and a non-conducting substrate isused, a film of an electrical conductor to serve as the cathodeelectrode and provide means to apply a voltage to the electron emittingmaterial will be needed. Silicon, a glass, a metal or a refractorymaterial such as alumina can serve as the substrate. For displayapplications, the preferable substrate is glass and soda lime glass isespecially preferred. For optimum conductivity on glass, silver pastecan be pre-fired onto the glass at 500-550° C. in air or in an inert gassuch as nitrogen, but preferably in air, or the substrate may be coatedwith a layer of ITO. The conducting layer so-formed can then beover-printed with the emitter paste.

The paste used for conventional screen printing typically contains theelectron emitting material, an organic medium, solvent, surfactant andeither low softening point glass frit, metallic powder or metallic paintor a mixture thereof. The role of the medium and solvent is to suspendand disperse the particulate constituents, i.e. the solids, in the pastewith a proper rheology for typical patterning processes such as screenprinting. There are many organic media known for use for such purposeincluding cellulosic resins such as ethyl cellulose and alkyd resins ofvarious molecular weights. Butyl carbitol, butyl carbitol acetate,dibutyl carbitol, dibutyl phthalate and terpineol are examples of usefulsolvents. These and other solvents are formulated to obtain the desiredviscosity and volatility requirements.

A glass frit that softens sufficiently at the firing temperature toadhere to the substrate and to the electron emitting material is alsoused. A lead or bismuth glass frit can be used as well as other glasseswith low softening points such as calcium or zinc borosilicates. If ascreen printable composition with higher electrical conductivity isdesired, the paste may also contain a metal, for example, silver orgold. The paste typically contains about 40 wt % to about 80 wt % solidsbased on the total weight of the paste. These solids include theelectron emitting material and glass frit and/or metallic components.Variations in the composition can be used to adjust the viscosity andthe final thickness of the printed material.

If the screen-printed paste is to be photopatterned, the paste may alsocontain a photoinitiator, a developable binder and a photohardenablemonomer comprised, for example, of at least one addition polymerizableethylenically unsaturated compound having at least one polymerizableethylenic group. Typically, a paste prepared from an electron emittingmaterial such as CNTs, silver and glass frit will contain about 0.01-6.0wt % nanotubes, about 40-75 wt % silver in the form of fine silverparticles and about 3-15 wt % glass frit based on the total weight ofthe paste.

The emitter paste is typically prepared by three-roll milling a mixtureof the electron emitting material, organic medium, surfactant, solventand either low softening point glass frit, metallic powder or metallicpaint or a mixture thereof. The paste mixture can be screen printedusing, for example, a 165-400-mesh stainless steel screen. The paste canbe deposited as a continuous film or in the form of a desired pattern.

After printing, the conventionally-prepared electron field emitter isfurther processed by removing any residual photoresist material, dryingthe plate, and then installing it as the anode plate in anelectrochemical cell. The cell is similar in construction to the celldescribed above, and the cathode therein may be stainless steel or anynon-oxidizable conductor. The electrolyte, which is disposed between thecathode and the anode, contains boric acid and/or a borate compound.This embodiment of the methods hereof is generally performed byoperation of the cell at a potential of less than about 10 volts, or inthe range of from about 2 to about 6 volts, or in the range of fromabout 3 volts to about 5 volts. The cell may be operated for a period oftime in the range of from about 1 to about 10 minutes, or in the rangeof from about 2 to about 6 minutes, or in the range of from about 3 toabout 5 minutes.

After completion of operation of the cell, the plate may be removed fromthe cell, rinsed, dried and installed in such condition in a fieldemission device for use as part of the cathode assembly therein toprovide electron emission in devices such as described above.Alternatively, however, before installation in a field emission device,the plate may first be baked and/or fired to melt the depositedpolymer(s) and utilize them in that form as an adhesive to more securelyanchor the CNTs to the surface of the plate, resulting in aCNT-containing film with excellent abrasion resistance. Firing may beperformed at a temperature in the range of about 250° C. to about 650°C., or about 350° C. to about 550° C., or about 450° C. to about 525°C., for a period of time in the range of about 5 to about 30 minutes, orabout 10 to about 25 minutes, or about 10 to about 20 minutes, innitrogen or air. Higher firing temperatures can be used with substratesthat can endure them provided the atmosphere is free of oxygen. However,the organic constituents in the paste are effectively volatilized at350-450° C., leaving a layer of the composite of the electron emittingmaterial and glass and/or metallic conductor.

In this embodiment of the methods hereof, where (as above) boric acidand/or a borate compound is present in the electrolyte solution, thematerial as deposited on the cell anode plate may thus include, inaddition to the CNT complexes, some of the boric acid and/or boratecompound. In such event, a further embodiment of this invention includesa film that is composed of a substrate and, disposed or deposited on thesubstrate, boric acid and/or a borate compound and a complex formed fromcarbon nanotubes and one or more anionic polymer(s).

In this embodiment, boric acid and/or a borate compound may be used inthe electrolyte solution at a concentration therein in the range ofabout 0.1 wt % or more, or about 0.5 wt % or more, and yet about 10 wt %or less, or about 5 wt % or less.

Materials as used in the process hereof may be made by processes knownin the art, or are available commercially from suppliers such as AlfaAesar (Ward Hill, Mass.), City Chemical (West Haven, Conn.), FisherScientific (Fairlawn, N.J.), Sigma-Aldrich (St. Louis, Mo.) or StanfordMaterials (Aliso Viejo, Calif.).

The advantageous attributes and effects of this invention may be seen ina series of examples (Examples 1˜5), as described below. The embodimentson which the examples are based are representative only, and theselection of those embodiments to illustrate the invention does notindicate that materials, conditions, specifications, components,reactants, techniques and protocols not described in these examples arenot suitable for practicing this invention, or that subject matter notdescribed in these examples is excluded from the scope of the appendedclaims and equivalents thereof.

Examples

150 mg of laser-ablated CNTs (from CNI, Houston, Tex.) was mixed with 30mg yeast RNA (from Sigma Aldrich) in 15 mL of 1× TBE [tris borate(ethylenediaminetetraacetic acid)] buffer (from Sigma Aldrich). Themixture was sonicated with a probe sonicator at a power level of 20 Wfor 30 min. The resulting dispersion (“CNT Dispersion”) was mixed withtwo other components according to the following table (Table 1) to makeup 100 mL of deposition solution. Ru²⁺(bipy)₃ as used in the depositionsolution is tris(2,2′-bipyridyl)dichloro-ruthenium (II) and is obtainedfrom Sigma Aldrich. EMMA is ethylene/methacrylic acid ionomer obtainedfrom DuPont as Surlyn™ ionomer.

TABLE 1 Composition of Deposition Solution Stock Volume Final Componentconcentration added conc. CNT 10 mg/mL 4 mL 0.04% Dispersion EMMA 10mg/mL 2 mL 0.02% Ru²⁺⁽bipy)₃ 10 mM   2 mL 0.2 mM water 92 mL 

Example 1

A photoresist (PR) patterned glass substrate coated with indium tinoxide (ITO) (2″×2″) (used as the cell anode) was prepared. The PR layerdefines an array of open circular wells with 20 μm diameter. The opencircular wells expose the ITO surface for CNT deposition. Beforeelectrodeposition, the PR coated ITO plate was dipped into a solution of0.01% Triton X-100 for 30 seconds, taken out and dried by blowing N₂gas. This step is to coat the hydrophobic PR layer with a thinhydrophilic layer for better wetting.

After this treatment, a 2″×2″ stainless steel plate (used as the cellcathode) and the PR-coated ITO plate (used as the cell anode) wereinserted in a parallel fashion into a rectangular cell containing 15 mLof the deposition solution. FIG. 2 shows the rectangular cell containingdeposition solution into which the stainless steel cathode and PR-coatedanode are inserted in a parallel fashion. The electrochemical cell isdesignated as number 1, the slot for the cathode is designated as number2 and the slot for the anode is designated as number 3.

A DC potential of 2.5 V (obtained from a Princeton Applied Research,Model 263A, Oak Ridge, Tenn.) was applied between the two electrodes.After 2 min, the deposition was stopped, and the ITO plate was taken outof the cell, rinsed with DI water and dried in air. The PR layer wasstripped off by organic solvents such as acetone or a NMP:H₂O solution.The cell anode was then rinsed in DI H₂O and dried under flowing N₂ gas.

Control samples were made using the same laser ablated carbon nanotubepowder from which the dispersion described above was derived. Thenanotube powder was incorporated into a paste and screen printed onto a2″×2″ PR patterned ITO substrate. After imaging under UV exposure, theprinted substrate was rinsed for 65 seconds in a NMP:H₂O solution.

Both the control and the electrochemically deposited substrates werefired in air in a 10-zone belt furnace (Lindberg, 810 thick-filmconveyor, Watertown, Wis.) to 400° C. peak for 21 minutes. Thesubstrates were then activated by placing an adhesive in contact withthe patterned surface. Each activated substrate was then incorporatedinto a diode device as the cathode, with a 620 μm spacer between the2″×2″ ITO coated phosphor glass substrate that served as the anode. Thediode thus formed was placed in a vacuum chamber evacuated to a basepressure below 1×10⁻⁵ Torr.

A negative voltage pulse with a pulse width of 60 us at 60 Hz wasapplied to each diode using an IRCO high voltage source (ModelF5k-10-02N, IRCO, Columbia Md.). The pulsing was supplied from a pulsegenerator (Stanford Research Systems, Inc., model DG535, Sunnyvale,Calif.). The resulting emission current was measured as a function ofapplied voltage using a Keithley 2000 multimeter (Keithley Instruments,Cleveland, Ohio). The field required to obtain 20 μA or more was noted.For the control sample, this field was found to be generally 4.5V/μm orgreater. For the electrochemically deposited sample, the field wasgenerally on the order of 2.5V/μm. FIG. 3 shows the average emissionfields from the samples that were made from an electrochemicaldeposition (ECD) technique (square) and from a screen-printing (non ECD)technique (circle). Lower operational fields are preferred.

Example 2

Carbon nanotube powder made from a laser ablation process wasincorporated into a thick film paste and screen printed onto a 2″×2″photoresist (PR) patterned glass substrate coated with indium tin oxide(ITO). The PR layer defines an array of open circular wells with 20 μmdiameter. The open wells expose the ITO surface onto which theCNT-containing paste can be screen printed. After imaging the printedsurface under UV exposure, the substrate was rinsed for 65 seconds in aNMP:H₂O solution to reveal the patterned structure.

A 2″×2″ stainless steel plate (used as the cell cathode) and the 2″×2″screen printed substrate on ITO (used as the cell anode) were insertedin a parallel fashion into a rectangular cell (as shown in FIG. 2)containing 15 mL of electrolyte solution (1× TBE or 0.1 M Boric acid,Sigma Aldrich). A DC potential of 3V (Princeton Applied Research, Model263A) was applied between the two electrodes. After 4 min, the treatmentwas stopped, and the ITO plate was taken out of the cell, and allowed todry in air.

The substrate (cell anode) was then fired in air to 400° C. peak for 21minutes in a 10-zone belt furnace (Lindberg, 810 thick-film conveyor,Watertown, Wis.). The substrate was then activated by placing anadhesive in contact with the patterned surface containing the carbonnanotube paste. The substrate was then incorporated into a diode deviceas the cathode, separated from the ITO coated phosphor glass anode by a620 μm spacer. The diode thus formed was placed in a vacuum chamberevacuated to a base pressure below 1×10⁻⁵ Torr.

A negative voltage pulse with a pulse width of 60 us at 60 Hz wasapplied using an IRCO high voltage source (Model F5k-10-02N, IRCO,Columbia, Md.). The pulsing was supplied from a pulse generator(Stanford Research Systems, Inc., Model DG535, Sunnyvale, Calif.). Theresulting emission current was measured as a function of applied voltageusing a Keithley 2000 multimeter (Keithley Instruments, Cleveland,Ohio). The field required to obtain 20 μA or more was noted.

For a control sample not subjected to electrochemical treatment, thisfield was found to be generally greater than 5V/μm. For theelectrochemically treated sample, the fields needed were generally onthe order of 2.5V/μm to 3.0V/μm. FIG. 4 shows the emission curves fromscreen printed samples that were either treated in an electrochemicalcell (solid lines) or not treated in an electrochemical cell (dottedlines). Lower operational fields for any given current are preferred.

Features of certain of the devices of this invention are describedherein in the context of one or more specific embodiments that combinevarious such features together. The scope of the invention is not,however, limited by the description of only certain features within anyspecific embodiment, and the invention also includes (1) asubcombination of fewer than all of the features of any describedembodiment, which subcombination may be characterized by the absence ofthe features omitted to form the subcombination; (2) each of thefeatures, individually, included within the combination of any describedembodiment; and (3) other combinations of features formed by groupingonly selected features of two or more described embodiments, optionallytogether with other features as disclosed elsewhere herein.

In this specification, unless explicitly stated otherwise or indicatedto the contrary by the context of usage, where an embodiment of thesubject matter hereof is stated or described as comprising, including,containing, having, being composed of or being constituted by or ofcertain features or elements, one or more features or elements inaddition to those explicitly stated or described may be present in theembodiment. An alternative embodiment of the subject matter hereof,however, may be stated or described as consisting essentially of certainfeatures or elements, in which embodiment features or elements thatwould materially alter the principle of operation or the distinguishingcharacteristics of the embodiment are not present therein. A furtheralternative embodiment of the subject matter hereof may be stated ordescribed as consisting of certain features or elements, in whichembodiment, or in insubstantial variations thereof, only the features orelements specifically stated or described are present.

1. A method for the deposition of carbon nanotubes, comprising: (a)providing an electrochemical cell that comprises a cathode, an anodeplate, a first electrically conducting pathway connecting the cathode toan electrical power supply, and a second electrically conducting pathwayconnecting the electrical power supply to the anode plate; (b) providingas an aqueous electrolyte disposed between the cathode and the anode adispersion of a complex formed from carbon nanotubes and a first anionicpolymer; (c) applying a voltage to the electrochemical cell to depositthe complex on the anode; and (d) removing the anode plate from theelectrochemical cell and firing the plate in air.
 2. A method accordingto claim 1 wherein the aqueous electrolyte further comprises acoagulant.
 3. A method according to claim 2 wherein coagulant residue isdeposited on the anode together with the complex.
 4. A method accordingto claim 1 wherein the first polymer comprises a nucleic acid molecule.5. A method according to claim 1 wherein the first polymer comprisesRNA.
 6. A method according to claim 1 wherein the electrolyte furthercomprises a second anionic polymer.
 7. A method according to claim 6wherein the second ionic polymer comprises a styrenic ionomer or anionized ethylene/(meth)acrylic acid
 8. A method according to claim 6wherein the complex, as deposited on the anode, is deposited in a matrixof the second anionic polymer.
 9. A method according to claim 7 whereinthe first polymer comprises a nucleic acid molecule.
 10. A methodaccording to claim 1 further comprising a step of removing the anodeplate from the cell, and installing it in a field emission device.
 11. Amethod for the deposition of carbon nanotubes, comprising: (a) providingan electrochemical cell that comprises a cathode, an anode plate, afirst electrically conducting pathway connecting the cathode to anelectrical power supply, and a second electrically conducting pathwayconnecting the electrical power supply to the anode plate; (b) providingan aqueous electrolyte disposed between the cathode and the anode,wherein the electrolyte comprises boric acid and/or a borate compound,and a dispersion of a complex formed from carbon nanotubes and a firstanionic polymer; and (c) applying a voltage to the electrochemical cellto deposit the complex on the anode.
 12. A method according to claim 11wherein boric acid and/or a borate compound is deposited on the anodetogether with the complex.
 13. A method according to claim 11 whereinthe aqueous electrolyte further comprises a coagulant.
 14. A methodaccording to claim 13 wherein coagulant residue is deposited on theanode together with the complex.
 15. A method according to claim 11wherein the first polymer comprises a nucleic acid molecule.
 16. Amethod according to claim 11 wherein the first polymer comprises RNA.17. A method according to claim 11 wherein the electrolyte furthercomprises a second anionic polymer.
 18. A method according to claim 17wherein the second ionic polymer comprises a styrenic ionomer or anionized ethylene/(meth)acrylic acid copolymer.
 19. A method according toclaim 17 wherein the first polymer comprises a nucleic acid molecule.20. A method according to claim 11 further comprising a step of removingthe anode plate from the cell, and installing it in a field emissiondevice.
 21. A film comprising a substrate and, disposed on thesubstrate, (a) boric acid and/or a borate compound, and (b) a complexformed from carbon nanotubes and a first anionic polymer. 22-28.(canceled)
 29. A cathode assembly for a field emission device comprisinga film according to claim
 21. 30. A field emission device comprising acathode assembly according to claim
 29. 31-35. (canceled)